Frequency-Dependent Selection and Heterozygote Advantage are two type of balance selection that contribute to maintaining certain phenotypes in an oscillating equilibrium.
In regions of tropical rainforests where toxic butterflies exist, there is often a presence of non-toxic butterflies that mimic the patterns of the toxic ones. These non-toxic butterflies deceive birds into perceiving them as toxic. The proportions of these butterflies constantly fluctuate within a certain range. When the population of non-toxic butterflies becomes too high, it misleads birds into perceiving a decrease in toxicity and increase predation. However, the low population of toxic butterflies reduces the probability of being captured, giving them a reproductive advantage. As the toxic butterflies reproduce in excess, the birds resort to eating other insects. This results in a decrease of selection pressure for non-toxic butterflies and their population becomes big again, initiating a new cycle of natural selection.
Sickle cell disease occurs in regions plagued by malaria. The gene A for hemoglobin is mutated to produce a recessive allele, gene a. Heterozygotes (Aa) can resist malaria without significant health issues, while homozygotes (AA and aa) are both unable to resist malaria. Homozygotes with aa also suffer from severe anemia, blood clots, and other diseases. Clearly, the Aa genotype is more advantageous for survival. The frequency of the a allele increases, but it does not completely eliminate the A allele like in directional selection. The recessive allele requires A gene to show its advantages. Furthermore, not all individuals with the AA genotype die from malaria. In fact, they have some resistance to the disease, and the presence of the a allele enhances this resistance. Eventually, the a allele will stabilize at a certain frequency.